Item 18 on the n7= line in mirc.ini's [options] section is the default editbox width in percentage (defaults to 70).
I.e., it is a user-configurable option. However I cannot find any means of adjusting this parameter through the mIRC GUI.
The default nicklist width is set by:
* adjusting a channel window's nicklist width by hand
* by then going to System Menu > Nick List > "[x] Set as Default"
So I would assume the default editbox width would similarly be set by:
* adjusting a channel window's editbox width by hand
* by then going to System Menu > Edit Box > "[x] Set as Default"
But this does not actually change n7= #18 (even after forcing a /saveini).